Optical head and image forming apparatus employing the same

ABSTRACT

Image data is inputted from a data processing device  23  into storage means  24  formed on a light-emitting element (yellow) line head  28  so that light emitting elements in one line  28   a  expose pixels on an image carrier in response to output signal from a shift resistor  24   a . As the image carrier is moved in the direction of X to bring said pixels to reach the position corresponding to light emitting elements in the next line  28   b , the image data is transmitted to a shift resistor  24   b  so that the shift resistor  24   b  outputs the image data to light emitting elements in the line  28   b , whereby the pixels are exposed to light again. The movement of the image carrier and the transmission of the image data to the respective shift resistors are sequentially conducted, thereby conducting multiple exposure of each same pixel.

BACKGROUND OF THE INVENTION

The present invention relates to an optical head of multi-exposure typecapable of giving gradation in which the circuit structure is simplifiedand the emission control is speeded up for exposing pixels on an imagecarrier to light and also relates to an image forming apparatusemploying the same.

In conventional image forming apparatus in which a latent image iswritten on an image carrier, it is common practice to employ a lightemitting diode (LED) array as writing means. In case of employing alight emitting elements such as LEDs, it is required to pay attention tothe relation between the luminance (amount of light) and the lifeduration of each light emitting element. That is, the life duration canbe increased by reducing the luminance of the light emitting element. Inthis case, however, there is a problem that the amount of light forexposure is insufficient to form image. When the luminance of the lightemitting element is increased, enough amount of light for exposure forforming image is obtained. In this case, however, there is a problemthat the life duration is shortened.

For this, the development of material for obtaining light emittingelements capable of providing large luminance and having long lifeduration has been encouraged. However, in the present state of affairs,it is too expensive to achieve the practical use. In this connection, aline head (optical head) of multi-exposure type in which each pixel isexposed to light by a plurality of light emitting elements. As examplesof such line head of multi-exposure type, (1) Japanese Patent UnexaminedPublication No. S61-182966 discloses a recording array head on whichlight recording elements are aligned in a plurality of lines relative tothe rotational direction of a photosensitive drum. Image data are formedagain and again on a same pixel by shifting the light recording elementsin the direction of the lines while moving the photosensitive drum. Theexample (1) has an advantage that high-speed image formation is achievedeven using light recording elements with low light-emitting output.

(2) Japanese Patent Unexamined Publication No. S64-26468 discloses an ELelement panel composed of a group of EL elements of 20 dots(vertical)×640 dots (horizontal). The EL element group is driven foreach line at the same speed of the moving speed of a photoreceptor.Accordingly, each pixel is irradiated by light of which amount istwentyfold of the amount of light of each EL element. Also in thisexample, the amount of light for exposure per pixel is increased,thereby achieving the speed-up of the image formation. In addition, (3)Japanese Patent Unexamined Publication No. H11-129541 discloses a printhead on which LEDs are aligned in a plurality of lines. Multipleexposure is made on each pixel by moving the print head in the mainscanning direction. In this example, since the multiple exposure isconducted, variations in amount of light among the respective LEDs canbe equalized, thereby improving the image quality. (4) Japanese PatentUnexamined Publication No. 2000-260411 discloses an optical printer headon which plural lines of LED array chips are aligned. The gradation ofeach pixel can be changed among three levels by turning ON or OFF theLED array chips on each line.

The techniques disclosed in the aforementioned (1), (2) relate tomonochrome image formation and have a problem that the gradation controlfor neutral density is impossible. Since the technique disclosed in theaforementioned (3) is of a serial type that the line head is driven,there is a problem of complexity of the driving mechanism. Since thetechnique disclosed in the aforementioned (4) is structured such thatthe LED array chips on each line are turned ON or OFF, there is aproblem of complexity of the control circuit. Since the number of lightemitting elements in the line head of multi-exposure type are greaterthan that of a line head of normal exposure type and it is necessary tocontrol the light emitting elements synchronously with the movement ofthe photoreceptor, there is a problem of complexity of the controlcircuit for conducting the data processing and it is difficult to speedup the emission control.

Especially, in case that a line head (optical head) of multi-exposuretype is employed for color image formation, the amount of data to beprocessed must be severalfold of that in the case of ON-OFF controlbecause the gradation control for each pixel is sometimes required. Thismakes the speed-up of emission control further difficult. In case of aline head of multi-exposure type, it is required to send a largequantity of data formed by a data processor to the line head.Accordingly, the number of wires between the line head and the body ofimage forming apparatus is increased and it is necessary to use aninterface capable of supporting the high-speed processing, thusincreasing the cost.

SUMMARY OF THE INVENTION

The present invention was made in view of the aforementioned problems ofconventional techniques and the object of the present invention is toprovide a optical head of multi-exposure type capable of givinggradation in which the circuit structure is simplified and the emissioncontrol is speeded up for exposing pixels on an image carrier to lightand to provide an image forming apparatus employing the same.

An optical head of a first aspect of the present invention achieving theaforementioned object comprises: light emitting elements which arealigned in a plurality of lines, extending in the main scanningdirection of an image carrier and being arranged in parallel with eachother in the sub scanning direction of the image carrier, so that saidlight emitting elements are arranged in two-dimensional array; andstorage means for storing image data and outputting the image data tosaid light emitting elements, wherein said light emitting elements andsaid storage means are mounted together, said optical head beingcharacterized in that pixels on said image carrier are exposed by thelight emitting elements aligned in one line and exposed again by thelight emitting elements aligned in the next line after the movement ofsaid image carrier, and in the same manner, said pixels are sequentiallyexposed by the light emitting elements in another line after themovement of said image carrier so as to achieve multiple exposure of thepixels, and characterized by being capable of exposing pixels withgradational outputs.

The optical head of the first aspect is characterized in that lightemitting elements on the respective lines which are aligned to expose asame pixel repeatedly expose said same pixel with the same amount oflight. The light emitting elements have the following features. That is,(1) the interval in the sub scanning direction between spot positionsformed on the image carrier by the light emitting elements is anintegral multiple of the pixel pitch in the sub scanning direction; (2)the light emitting elements are controlled by a driving circuitaccording to the active matrix method; (3) the amount of light of thelight emitting elements are controlled in the PWM method; (4) the amountof light of the light emitting elements are controlled in the intensitymodulation method; (5) each of said light emitting elements comprises anorganic EL.

In the optical head of the first aspect, the storage means have thefollowing features. That is, (1) each of the storage means comprises aTFT; (2) the storage means are arranged to correspond to the lines ofthe light emitting elements, respectively and are designed to transportimage data, hold the image data, and output the image data to the lightemitting elements; (3) the storage means are formed on a same substrateon which the light emitting elements are formed; and (4) there are linesof pixels to be exposed and lines of pixels not to be exposed on saidimage carrier, the light emitting elements on the respective lines arearranged to correspond to the lines of pixels to be exposed,respectively, said storage means are arranged to correspond to both thelines of pixels to be exposed and the lines of pixels not to be exposed,respectively, and the storage means corresponding to the lines of pixelsnot to be exposed do not output said image data.

An image forming apparatus of a first aspect of the present inventionachieving the aforementioned object is of a tandem type and comprises atleast two image forming stations each having an image carrier andfurther having a charging means, an exposure head, a developing means,and a transfer means which are arranged around said image carrier andforming a color image by passing a transfer medium through therespective stations, wherein said exposure head comprises: lightemitting elements which are aligned in a plurality of lines, extendingin the main scanning direction of an image carrier and being arranged inparallel with each other in the sub scanning direction of the imagecarrier, so that said light emitting elements are arranged intwo-dimensional array; and storage means for storing image data andoutputting the image data to said light emitting elements and saidexposure head is an optical head on which said light emitting elementsand said storage means are mounted together, said optical head beingcharacterized in that pixels on said image carrier are exposed by thelight emitting elements aligned in one line and exposed again by thelight emitting elements aligned in the next line after the movement ofsaid image carrier, and in the same manner, said pixels are sequentiallyexposed by the light emitting elements on another line after themovement of said image carrier so as to achieve multiple exposure of thepixels, and characterized by being capable of exposing pixels withgradational outputs. The image forming apparatus of the first aspect ischaracterized in that an optical head having any one of theaforementioned features is employed as the optical head.

In the optical head of the first aspect of the present invention, thestorage means are provided on the line head together with the lightemitting elements. Therefore, the data amount to be sent from the imageforming apparatus to the line head can be reduced, thereby reducing thenumber of wires between the image forming apparatus and the line head.

The storage means of the present invention are formed on the samesubstrate on which the light emitting elements are formed. Therefore,the light emitting elements and the storage means can be manufacturedtogether. The necessity of preparing the light emitting elements and thestorage means on separate chips can be eliminated, thereby reducing themanufacturing cost. Further, each storage means comprises a TFT in thepresent invention. Since the light emitting elements and storageelements are formed on the single substrate such as a glass substratehaving high dimensional stability, improvement in accuracy of aiminglight from each light emitting element to each pixel on the imagecarrier is achieved.

In the optical head of the first aspect of the present invention, oncedata only for one line, i.e. the first line, is produced, the image datafor the first line is stored in the storage means (shift resistor) andare transmitted among the storage means, where by the operations of alllight emitting elements of the optical head can be controlled. Since itis not required to produce data for all light emitting elements of theoptical head, the structure of circuit can be simplified and the dataprocessing can be conducted at high speed.

Further, in the optical head of the first aspect of the presentinvention, storage means for pixel line and lines of light emittingelements can be arranged in one-on-one relation. Therefore, the timingfor transmitting image data stored in a storage means to the nextstorage means and the timing for making light emitting elements in theline to emit light on the basis on the image data for pixel line storedin the storage means can be synchronized, thereby simplifying thecircuit structure and speeding up the operation of the light emittingelements.

Furthermore, in the optical head of the first aspect of the presentinvention, the light emitting elements are controlled in the activematrix method. Accordingly, the light emitting elements can bemaintained to keep emitting light by means of condensers and transistorsarranged around the light emitting elements. Therefore, the lightemitting elements remain to emit light even during the transmission ofimage data from a storage means to the next storage means, therebyexposing pixels with high luminance. In addition, in the optical head ofthe first aspect of the present invention, the amount of light emittedfrom the light emitting elements is controlled in the PWM method. Sincethe amount of exposure can be changed by ON/OFF of the light emittingelements, the circuit structure can be simplified.

Moreover, in the optical head of the first aspect of the presentinvention, the amount of light emitted from the light emitting elementsis changed in the intensify modulation method. Therefore, it is notrequired to control the ON/OFF of the light emitting elements at a highspeed. Even when the speed of response of the light emitting elements isslow, the amount of exposure can be changed at a high speed. Inaddition, in the optical head of the first aspect of the presentinvention, each light emitting element is composed of an organic EL.Therefore, the light emitting elements can be easily formed on a glasssubstrate, thereby achieving lower price.

An optical head of a second aspect of the present invention achievingthe aforementioned object comprises: light emitting elements which arealigned in a plurality of lines, extending in the main scanningdirection of an image carrier and being arranged in parallel with eachother in the sub scanning direction of the image carrier, so that saidlight emitting elements are arranged in two-dimensional zigzag fashion;and storage means for storing image data and outputting the image datato said light emitting elements, said optical head being characterizedin that pixels on said image carrier are exposed by the light emittingelements aligned in one line and exposed again by the light emittingelements aligned in the next line after the movement of said imagecarrier, and in the same manner, said pixels are sequentially exposed bythe light emitting elements on another line after the movement of saidimage carrier so as to achieve multiple exposure of the pixels, andcharacterized by being capable of exposing pixels with gradationaloutputs.

The optical head of the second aspect is characterized in that lightemitting elements on the respective lines which are aligned to expose asame pixel repeatedly expose said same pixel with the same amount oflight. The light emitting elements arranged in two-dimensional zigzagfashion have the following features. That is, (1) the interval in thesub scanning direction between spot positions formed on the imagecarrier by the light emitting elements is an integral multiple of thepixel pitch in the sub scanning direction; (2) the light emittingelements are controlled by a driving circuit according to the activematrix method; (3) the amount of light of the light emitting elementsare controlled in the PWM method; (4) the amount of light of the lightemitting elements are controlled in the intensity modulation method; (5)each of said light emitting elements comprises an organic EL.

In the optical head of the second aspect, pixels to be exposed by lightemitting elements and pixels not to be exposed by light emittingelements are arranged in the zigzag fashion in said sub scanningdirection on said image carrier, and the storage means comprise firststorage means each of which outputs image data to the light emittingelements in the corresponding line and second storage means each ofwhich transfer the image data to the next first storage means withoutoutputting the image data to the light emitting elements.

In the optical head of the second aspect, the storage means foroutputting the light emitting elements arranged in two-dimensionalzigzag fashion have the following features. That is, (1) the storagemeans are formed on a same substrate on which the light emittingelements are formed; (2) each of the storage means comprises a TFT; and(3) the storage means are arranged to correspond to the lines of thelight emitting elements, respectively and are designed to transportimage data, hold the image data, and output the image data to the lightemitting elements.

An image forming apparatus of a second aspect of the present inventionachieving the aforementioned object is of a tandem type and comprises atleast two image forming stations each having an image carrier andfurther having a charging means, an exposure head, a developing means,and a transfer means which are arranged around said image carrier andforming a color image by passing a transfer medium through therespective stations, wherein said exposure head comprises: lightemitting elements which are aligned in a plurality of lines, extendingin the main scanning direction of an image carrier and being arranged inparallel with each other in the sub scanning direction of the imagecarrier, so that said light emitting elements are arranged intwo-dimensional zigzag fashion; and storage means for storing image dataand outputting the image data to said light emitting elements and saidexposure head is an optical head on which said light emitting elementsand said storage means are mounted together, said optical head beingcharacterized in that pixels on said image carrier are exposed by thelight emitting elements aligned in one line and exposed again by thelight emitting elements aligned in the next line after the movement ofsaid image carrier, and in the same manner, said pixels are sequentiallyexposed by the light emitting elements on another line after themovement of said image carrier so as to achieve multiple exposure of thepixels, and characterized by being capable of exposing pixels withgradational outputs. The image forming apparatus of the second aspect ischaracterized in that light emitting elements and storage means havingany one of the aforementioned features of the optical head of the secondaspect are employed.

In the optical head of the second aspect of the present invention inwhich the light emitting elements are arranged in the two-dimensionalzigzag fashion in said sub scanning direction on said image carrier,once data only for one line, i.e. the first line, is produced, the imagedata for the first line is stored in the storage means (shift resistor)and are transmitted among the storage means, whereby the operations ofall light emitting elements of the optical head can be controlled. Sinceit is not required to produce data for all light emitting elements ofthe optical head, the structure of circuit can be simplified and thedata processing can be conducted at high speed. In addition, the dataamount to be sent from the image forming apparatus to the optical headcan be reduced, thereby reducing the number of wires between the imageforming apparatus and the optical head.

Further, in the optical head of the second aspect of the presentinvention, the light emitting elements arranged in the zigzag fashionare controlled in the active matrix method. Accordingly, the lightemitting elements can be maintained to keep emitting light by means ofcondensers and transistors arranged around the light emitting elements.Therefore, the light emitting elements remain to emit light even duringthe transmission of image data from a storage means to the next storagemeans, thereby exposing pixels with high luminance. In addition, in theoptical head of the second aspect of the present invention, the amountof light emitted from the light emitting elements arranged in the zigzagfashion is controlled in the PWM method. Since the amount of exposurecan be changed by ON/OFF of the light emitting elements, the circuitstructure can be simplified.

Furthermore, in the optical head of the second aspect of the presentinvention, the amount of light emitted from the light emitting elementsarranged in the zigzag fashion is controlled in the intensify modulationmethod. Therefore, it is not required to control the ON/OFF of the lightemitting elements at a high speed. Even when the speed of response ofthe light emitting elements is slow, the amount of exposure can bechanged at a high speed. In addition, in the optical head of the secondaspect of the present invention, each of the light emitting elementsarranged in the zigzag fashion is composed of an organic EL. Therefore,the light emitting elements can be easily formed on a glass substrate,thereby achieving lower price.

In the optical head of the second aspect of the present invention, thestorage means are provided on the optical head together with the lightemitting elements. Therefore, the data amount to be sent from the imageforming apparatus to the line head can be reduced, thereby reducing thenumber of wires between the image forming apparatus and the line head.In addition, the storage means of the optical head of the second aspectaccording to the present invention are formed on the same substrate onwhich the light emitting elements arranged in the zigzag fashion areformed. Therefore, the light emitting elements and the storage means canbe manufactured together. The necessity of preparing the light emittingelements and the storage means on separate chips can be eliminated,thereby reducing the manufacturing cost.

Further, in the optical head of the second aspect of the presentinvention, said substrate is made of glass and each storage meanscomprises a TFT. Since the light emitting elements and storage elementsare formed on the single substrate such as a glass substrate having highdimensional stability, improvement in accuracy of aiming light from eachlight emitting element to each pixel on the image carrier is achieved.In addition, in the optical head of the second aspect of the presentinvention, storage means for pixel line and lines of light emittingelements can be arranged in one-on-one relation. Therefore, the timingfor transmitting image data stored in a storage means to the nextstorage means and the timing for making light emitting elements in theline to emit light on the basis on the image data for pixel line storedin the storage means can be synchronized, thereby simplifying thecircuit structure and speeding up the operation of the light emittingelements.

An image forming apparatus of a third aspect of the present inventionachieving the aforementioned object comprises: a plurality of lines eachof which is composed of a plurality of light emitting elements and whichare arranged in parallel with each other in the sub scanning directionof an image carrier so that said light emitting elements are arranged intwo-dimensional array, wherein pixels on said image carrier are exposedby the light emitting elements aligned in one line and exposed again bythe light emitting elements aligned in the next line after the movementof said image carrier, and in the same manner, said pixels aresequentially exposed by the light emitting elements in another lineafter the movement of said image carrier so as to achieve multipleexposure of the pixels, further comprises a control means forcontrolling said light emitting elements on the respective lines whichare aligned to expose same pixel such that said light emitting elementsexpose said same pixel with the same amount of light, and ischaracterized by being capable of exposing pixels with gradationaloutputs prepared by said control means.

Still other objects and advantages of the invention will in part beobvious and will in part be apparent from the specification.

The invention accordingly comprises the features of construction,combinations of elements, and arrangement of parts which will beexemplified in the construction hereinafter set forth, and the scope ofthe invention will be indicated in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram partially showing an example of an imageforming apparatus according to an embodiment of the present invention;

FIG. 2 is a block diagram showing the entire structure of the apparatusof FIG. 1;

FIG. 3 is an explanatory diagram showing an image forming apparatusaccording to another embodiment of the present invention;

FIG. 4 is a block diagram showing a control unit of the image formingapparatus;

FIG. 5 is a block diagram showing a control unit of an image formingapparatus according to another embodiment of the present invention;

FIG. 6 is a circuit diagram showing a control circuit for light emittingelements to be driven by active matrix method;

FIG. 7 is a table for explanation of an example of the relation betweenbit data and gradation data;

FIG. 8 is a block diagram of an example in which light emitting elementsare controlled by the pulse-width modulation (PWM) method;

FIG. 9 is an explanatory diagram of an example in which the lightemitting elements are controlled by the PWM method;

FIG. 10 is a block diagram of an example in which the light emittingelements are controlled by the intensity modulation method;

FIG. 11 is an explanatory diagram showing an image forming apparatusaccording to another embodiment of the present invention;

FIG. 12 is a block diagram showing a control unit according to ananother image forming apparatus of the present invention;

FIG. 13 is a block diagram showing a control unit according to ananother image forming apparatus of the present invention;

FIG. 14 is a block diagram showing a control unit according to ananother image forming apparatus of the present invention;

FIG. 15 is a perspective view showing an example of organic EL arraysaccording to an embodiment of the present invention;

FIG. 16 is a sectional view showing the schematic structure of theorganic EL arrays; and

FIG. 17 is a front view showing the schematic structure of an imageforming apparatus of a tandem type in which the organic EL array head ofthe present invention is arranged.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described in the below with reference tothe drawings. FIG. 2 is a block diagram showing the schematic structureof an optical head to be employed in an image forming apparatus of thepresent invention. In FIG. 2, a host computer 21 produce printing dataand sends the printing data to a control unit 22 of the image formingapparatus. The control unit 22 of the image forming apparatus comprisesa data processing means 23, storage means 24-27, and light-emittingelement line heads (optical heads) 28-31 arranged corresponding to theaforementioned storage means 24-27. The light-emitting element lineheads 28-31 correspond to four colors, i.e. yellow, magenta, cyan, andblack, respectively, to form color images on a photoreceptor. Thestorage means 24-27 store image data corresponding to light-emittingelement line heads 28-31 for the respective colors.

The data processing means 23 carries out processes such as colorseparation, gradation treatment, bit-mapping of image data, andcorrection of color registration error. The data processing means 23outputs image data for each line to each storage means 24-27. Eachlight-emitting element line head 28-31 has a plurality of light emittingelement lines arranged therein and is structured to conduct multipleexposure in which light emitting elements on the respective lines emitlight to a same pixel. Therefore, each storage means 24-27 outputs imagedata for plural lines to each light-emitting element line head 28-31.

FIG. 1 is a block diagram partially showing the structure shown in FIG.2. Shown in FIG. 1 are the light-emitting element (yellow) line head 28and the storage means 24 corresponding to the line head 28. In theexample shown in FIG. 1, the line head 28 has a line 28 a provided witha plurality of light-emitting elements 32. In this example, five lines28 a-28 e are arranged in the sub scanning direction X of an imagecarrier and each line has the same number of light-emitting elements.The storage means 24 comprise shift resistors 24 a-24 e to correspond tothe lines 28 a-28 e composed of the light-emitting elements,respectively. In FIG. 1, the direction of arrow X indicates the movingdirection (sub scanning direction) of a photosensitive drum (imagecarrier) and the direction of arrow Y indicates the main scanningdirection. Now, the operation of the block diagram shown in FIG. 1 willbe described. As the image data is inputted from the data processor 23into the storage means 24, the shift resistor 24 a outputs image data tothe light emitting elements in the first line 28 a so that the lightemitting elements work, whereby pixels on the image carrier are exposedto a predetermined amount of light.

The image carrier is driven to rotate in the direction of arrow X insuch a manner that the pixels exposed by the light emitting elements ofthe first line 28 a reach a position corresponding to the light emittingelements arranged in the next line 28 b. At the same time, the imagedata inputted in the shift resistor 24 a are transmitted to the shiftresistor 24 b. The shift resistor 24 b outputs the image data to thelight emitting elements of the line 28 b so that the light emittingelements work. Accordingly, the pixels previously exposed by the lightemitting elements of the line 28 a are exposed again by the lightemitting elements of the line 28 b with the equal amount of light.

In this manner, the image data is sequentially transmitted from theprevious shift resistor to the next shift resistor while the imagecarrier is moved in the direction of arrow X, whereby each same pixel isexposed again and again by light emitting elements in different lines.Consequently, in the example of FIG. 1, the respective pixels areexposed to light of which amount is quintuple of that of a single lightemitting element, thereby quickly obtaining the amount of light requiredto expose each pixel. The number of the lines in which the lightemitting elements are aligned in the sub scanning direction can besuitably selected, that is, the number for multiplying the amount oflight for exposure to be obtained by a single light emitting element canbe suitably selected, if necessary.

In case of gradation control for neutral density is conducted by thestructure of FIG. 1, assuming that the predetermined luminance is 1,image data for luminance of 0.1 are inputted from the data processor 23to the shift resistor 24 a. As mentioned above, by transmitting theimage data sequentially to the shift resistors 24 a-24 e to output theimage data to the light emitting elements while moving the imagecarrier, the luminance of each pixel becomes 0.1×5=0.5, providing aneutral density. In this manner, output for gradation when exposingpixels can be obtained. In the present invention, once the dataprocessing means 23 of the image forming apparatus produces data onlyfor one line, the image data for the first line is stored in the storagemeans (shift resistor) and are transmitted among the storage means,whereby the operations of all light emitting elements of the line headcan be controlled.

Since the data processing means is not required to produce data for alllight emitting elements of the line head, the structure of circuit canbe simplified and the data processing can be conducted at high speed. Inthe example of FIG. 1, the shift resistors 24 a-24 e functioning asstorage means are mounted on the line head 28 as well as the lightemitting elements. Therefore, the amount of data to be transmitted fromthe data processing means (control means) to the line head can bereduced, all that is required between the data processing means 23 andthe line head is only a single wiring 23 a.

In the example of FIG. 1, the shift resistors 24 a-24 e are formed on asame substrate on which the light emitting elements are formed.Therefore, the light emitting elements and the storage means can bemanufactured together. The necessity of preparing the light emittingelements and the storage means on separate chips can be eliminated,thereby reducing the manufacturing cost. Further, in the example of FIG.1, a glass is used as the aforementioned substrate so that the storagemeans are composed of Thin Film Transistors (TFTs). There are variousmethods of making the TFT. For example, silicon oxide is first depositedinto a layer on a glass substrate and amorphous silicon is thendeposited into a layer thereon. The amorphous silicon layer is exposedto excimer laser beam for crystallization, thus forming a polysiliconlayer as a channel.

After patterning the polysilicon layer, a gate insulating layer isformed and further a gate electrode is formed of tantalum nitride.Subsequently, source/drain regions for N-channel TFT are formed by ionimplantation of phosphorous and source/drain regions for P-channel TFTare formed by ion implantation of boron. After impurities of ionimplantation is activated, a first layer insulating film is formed,first contact holes are formed, source lines are formed, a second layerinsulating film is formed, second contact holes are formed and metallicpixel electrodes are formed, thereby completing the array of TFT (forexample, see “Polymer Organic EL Display” presented at the 8^(th)Electronic Display Forum (Apr. 18, 2001)).

The metallic pixel electrode is made of metal such as Mg, Ag, Al, and Liand functions as a cathode for the organic EL emitter and also as areflection layer for the organic EL emitter. Since the glass substratehaving high dimensional stability is structured and all of lightemitting elements and storage elements are formed on the singlesubstrate, improvement in accuracy of aiming light from each lightemitting element to each pixel on the image carrier is achieved.

FIG. 3 shows the structure of an optical head according to anotherembodiment of the present invention and is an explanatory diagram ofspot positions 33 to be formed on the image carrier. Hatched portions inFIG. 3 are spot positions. Pixels at these positions are exposed tolight. Positions indicated by chain double-dashed lines are pixels notto be exposed to light. “Pa” indicates a pixel pitch in the mainscanning direction and “Pb” indicates a pixel pitch in the sub scanningdirection. “S” indicates a pitch between spot positions (spot positionpitch) in the sub scanning direction which is an integer multiple of thepixel pitch. In this example, the pitch is twice as the pixel pitch. Asfor the spot positions 33 shown in FIG. 3, spots are formed in each oflines 33 a, 33 c, 33 e, 33 g, and 33 i on the image carrier by lightemitted from the light emitting elements, thereby exposing pixels. Ineach of lines 33 b, 33 d, 33 f, and 33 h, spots are not formed on theimage carrier by light emitted from the light emitting elements.

FIG. 4 is a block diagram corresponding to FIG. 3. In the example ofFIG. 4, a storage means 24 and a line head 28X are formed on a samesubstrate. Description will be made as regard to the light-emittingelement line heads 28X for yellow. The spot positions 33 shown in FIG. 3are formed by lines 28 f-28 n on which light emitting elements arealigned. Positions where no line of light emitting elements is formed inthe line head 28X of FIG. 4 correspond to the positions where no pixelis exposed shown in FIG. 3. The storage means 24 comprises a first groupconsisting of shift resistors 24 f-24 n corresponding to the lines 28f-28 n in which the light emitting elements are aligned, respectively.There is a second group consisting of shift resistors 24 g-24 m each ofwhich is arranged between each pair of adjacent shift resistors amongthe aforementioned shift resistors 24 f-24 n. The shift resistors 24g-24 m of the second group operates only for transmission of image datato the next shift resistor without outputting the image data to lightemitting elements.

In the example of FIG. 3 and FIG. 4, the operation for exposing pixelsat the spot positions 33 a in lines on the image carrier will bedescribed. The image data is outputted from the shift resistor 24 f tothe first line 28 f of light emitting elements, whereby pixels on theimage carrier are exposed. At the moment that the image carrier isdriven to rotate just for the pixel pitch Pb in the sub scanningdirection, the image data are transmitted from the shift resistor 24 fto the shift resistor 24 g. At this time, the shift resistor 24 g doesnot output the image data so that no pixel is exposed. At the momentthat the image carrier is further driven to rotate just for the pixelpitch in the sub scanning direction, the image data are transmitted fromthe shift resistor 24 g to the shift resistor 24 h. The shift resistor24 h outputs the image data to the light emitting elements of the line28 h. The light emitting elements on the line 28 h emit light to exposethe same pixels on the line in the spot positions 33 a.

In the same manner, the movement of the image carrier, the transmissionof the image data to the respective shift resistors, and the output ofthe image data to the light emitting elements are sequentiallyconducted, whereby each same pixel is exposed repeatedly. In this case,the gradation control for neutral density can be conducted on the basisof the data prepared by the data processing means 23.

Though the lines conducting exposure to pixels and the lines notconducting exposure to pixels are arranged alternatively every line inthe example of FIG. 3, two lines not conducting exposure may be arrangedbetween the lines conducting exposure. That is, the exposure to pixelsis conducted on every third line. In this case, shift resistors in twolines (arranged vertically nearby) are shift resistors which transmitimage data without outputting the image data to the corresponding linesand a shift resistor in the next (third) line conducts the control oflight emitting elements. In this manner, the present invention canprovide a variety of image formation on the image carrier.

According to the present invention, even when the interval in the subscanning direction between the spot positions where the light emittingelements emit light to the image carrier is an integral multiple of thepixel pitch in the sub scanning direction, the multiple exposure of eachpixel can be achieved by arranging the respective shift resistors tocorrespond to the line with light emitting elements and line withoutlight emitting elements as shown in FIG. 3 and FIG. 4. In this case, thetiming for transmitting image data stored in a shift resistor to thenext shift resistor and the timing for making light emitting elements inthe line to emit light on the basis on the image data stored in theshift resistor are synchronized, thereby simplifying the circuitstructure and speeding up the operation.

Though the spot position pitch in the sub scanning direction is twice asthe pixel pitch in the example of FIG. 3, the spot position pitch may heother integral multiple of the pixel pitch. Therefore, the spot positionpitch may be the same as the pixel pitch. In this case, the multiplenumber is 1.

FIG. 5 is a block diagram showing an image forming apparatus accordingto another embodiment of the present invention. The example shown inFIG. 5 is an apparatus in which light emitting elements are driven inthe active matrix method. In FIG. 5, “Z” indicates each single lightemitting part composed of a light emitting element and a driving circuitarranged according to the active matrix method. Five lines of lightemitting elements 28 p-28 t are arranged in a line head 28Y.Corresponding to the light emitting elements 28 p-28 t of the respectivelines, shift resistors 24 p-24 t are arranged. Connected to a dataprocessing device 23 is a line selector 34.

“35 a” designates a supply line of image data from the data processingdevice 23 to the shift resistors, “35 b” designates a control lineconnecting the data processing device 23 and the line selector 34, “36a-36 e” designate command lines for commanding action from the lineselector 34 to the respective shift resistors 24 p-24 t, “37 a-37 e”designate scanning lines for supplying signals from the line selector 34to the light emitting elements of the respective lines, and “38 a-38 k”designate signal lines for supplying operational signals from the shiftresistors 24 p-24 t to individual light emitting elements (organic ELs)in each line.

Description will now be made as regard to the operation of FIG. 5.According to a control signal supplied from the data processing device23 through the control line 35 b, the line selector 34 selects ascanning line 37 a and send a signal to the line of light emittingelements 28 p. In addition, the line selector 34 activates the shiftresistor 24 p according to the signal through the command line 36 a. Theshift resistor 24 p activates the signal lines 38 a-38 k to send outputsignals of image data to all of the light emitting elements 28 p in theline. The light emitting elements 28 p in the line emit lights to exposepixels. By changing the scanning line 37 and the command line 36according to the signal from the line selector 34, the above actions arealso conducted for the light emitting elements 28 q, 28 r, 28 s, and 28t, whereby the light emitting elements in all lines are activated toemit light to expose the pixels.

Then, the image data in the shift resistor 24 s is transmitted to theshift resistor 24 t. In the same manner, the image data is sequentiallytransmitted from the shift resistor 24 r to the shift resistor 24 s,from the shift resistor 24 q to the shift resistor 24 r, and the shiftresistor 24 p to the shift resistor 24 q. To the shift resistor 24 p,image data is transmitted from the data processing means 23 through thesignal line 35 a. During this, the image carrier is moved for the pixelpitch.

Since the light emitting elements at the light emitting parts Z remainto emit light because of the function of the active matrix, the lightemitting elements do not lights out even during the transmission ofimage data between the shift resistors, thereby exposing pixels withhigh luminance. By repeating the outputting of image data from the shiftresistor 24 to the light emitting elements, the transmission of theimage data between the shift resistors, and the movement of the imagecarrier, thereby consecutively writing the image data onto the imagecarrier.

FIG. 6 is a circuit diagram for operating the light emitting parts Zaccording to the active matrix. In FIG. 6, an organic EL is employed aseach light emitting element, “K” designates a cathode terminal thereofand “A” designates an anode terminal. The cathode terminal K isconnected to a power source which is not shown. “37 a” designates ascanning line which is connected to a gate Ga of a switching TFT (Tr1).“38 a” designates a signal line which is connected to a drain Da of theswitching TFT. “39” designates a power line and “Ca” designates astorage capacitor. A source Sb of a driving TFT (Tr2) of the organic ELis connected to the power line 39 and a drain Db is connected to theanode terminal A of the organic EL. In addition, a gate Gb of thedriving TFT is connected to a source Sa of the switching TFT.

Description will now be made as regard to the operation of the circuitshown in FIG. 6. As the signal line 38 a is energized in a state that avoltage of the power line 39 is applied to the source of the switchingTFT, the switching TFT is turn ON. Accordingly, the gate voltage of thedriving TFT is lowered and the voltage of the power line 39 is suppliedfrom the source of the driving TFT so that the driving TFT becomes tothe conducting state. As a result, the organic EL is activated to emitlight of a predetermined luminance. In addition, the storage capacitorCa is charged by the voltage of the power line 39.

Even when the switching TFT is turned OFF, the driving TFT is still inthe conducting state according to the charge stored in the storagecapacitor Ca so that the organic EL remains to emit light. Therefore, byadopting the active matrix to the driving circuit for the light emittingelements, the operation of the organic EL is maintained to keep emittinglight even when the switching TFT is turned OFF for transmitting theimage data between the shift resistors, thereby exposing pixels withhigh luminance.

In the present invention, by controlling the light emitting elements inthe pulse-width modulation (PWM) method to control the amount ofemitting light. By the control according to the PWM method, thegradation control for the light emitting elements can be achieved. Inthe present invention, gradation data is formed by an 8-bit gradationdata memory. FIG. 7 is a table for explanation of an example of therelation between bit data and gradation data. In the example of FIG. 7,the bit data No. 1 is a gradation data 0 (no light emission), the bitdata No. 8 is a data of the most condensed density, and the bit data No.2-No. 7 are data of neutral densities therebetween.

FIG. 8 is a block diagram of an example for conducting PWM control. InFIG. 8, a PWM control unit 70 is provided with gradation data memories71 a, 71 b . . . which may comprise shift resistors, a counter 72,comparators 73 a, 73 b . . . , and light emitting parts Za, Zb . . . .Supplied to the gradation data memories 71 a, 71 b . . . is a gradationdata signal 74 from, for example, the data processing means 23 shown inFIG. 7. The gradation data memories 71 a, 71 b . . . are 8-bit memoriesas shown in FIG. 7. The counter 72 counts reference clock signal 75.

The bit number of the counter 72 is eight bit the same as that of thegradation data memories 71 a, 71 b . . . so that the count repeats 0→the maximum (255)→0→ the maximum. The comparators 73 a, 73 b compare thesignal of the counter 72 to the gradation data stored in the gradationdata memories 71 a, 71 b . . . . When the gradation data>the countervalue, the switching TFT is turned OFF as shown in FIG. 6. When thegradation data≦the counter value, the switching TFT is turned OFF.

FIG. 9 shows a characteristic graph showing a concrete example of thePWM control shown in the block diagram of FIG. 8. The top graph of FIG.9 shows the output Da of the counter 72 which repeats 0→ the maximum(255)→0→ the maximum . . . as described in the above. The middle graphof FIG. 9 shows the waveform Db of the signal outputted from thecomparator, i.e. the operating characteristics of the switching TFT,when the gradation data is the bit data No. 7 (128 gradation level). Inthis case, the switching TFT is turned ON when the output of the counteris in a range from 0 to 127, and the switching TFT is turned OFF whenthe output of the counter is 128 and 255.

The bottom graph of FIG. 9 shows the waveform Dc of the signal outputtedfrom the comparator, i.e. the operating characteristics of the switchingTFT, when the gradation data is the bit data No. 6 (64 gradation level).In this case, the switching TFT is turned on when the output of thecounter is in a range from 0 to 63, and the switching TFT is turned OFFwhen the output of the counter is 64 and 255. In case of the middlegraph of FIG. 9, the pulse width of the waveform Db is Wa. In case ofthe middle graph of FIG. 9, the pulse width of the waveform Dc is Wb.That is, according to the size of the gradation data, the time periodfor which the switching TFT is turned ON is changed, thereby changingthe amount of light emitted from the light emitting elements. Since theamount of exposure to the image carrier can be changed by ON/OFF of thelight emitting elements according to the ON/OFF control of the switchingTFT, the circuit structure can be simplified.

FIG. 10 is a block diagram showing another structure according to thepresent invention. The same pats as used in FIG. 8 are marked with thesame numerals or marks, so the detail description about such parts willbe omitted. The example shown in FIG. 10 controls the switching TFT withvoltages or currents corresponding to the sizes of the gradation data.Such control as shown in FIG. 10 is called “Intensity Modulation” in thepresent invention. In an intensity modulation control unit 80 shown inFIG. 10, D/A converters 81 a, 81 b . . . are connected to the gradationdata memories 71 a, 71 b . . . , respectively. The D/A converters 81 a,81 b . . . form voltage values of analog or current values correspondingto the sizes of the gradation data stored in the gradation data memories71 a, 71 b . . . and output the voltage values or current values to theswitching TFTs, respectively.

In the example of FIG. 10, the amount of light emitted from the lightemitting elements is changed by changing the bias of the switching TFTcorresponding to the gradation data. Therefore, it is not required tocontrol the ON/OFF of the light emitting elements at a high speed. Evenwhen the speed of response of the light emitting elements is slow, theamount of exposure to the image carrier can be changed at a high speed.Light emitting parts Za, Zb . . . are driven in the active matrix methodshown in FIG. 6. Supplied to the light emitting parts Za, Zb . . . are aselect signal through a scanning line 37 a and control signals throughemission control data lines 38 a, 38 b. . . .

By the way, in arranging plural light emitting elements in each line inthe image forming apparatus, it is structurally impossible to arrangeadjacent light emitting elements with no space therebetween. When lightemitting elements are arranged to correspond to the line 33 a of thespot positions in FIG. 4 for example, spaces are created betweenadjacent light emitting elements. Because of these spaces, theresolution is reduced and defect of transferred toner occurs when theimage data is recorded on a recording medium, leading to deteriorationin quality. To avoid this, the light emitting elements are arranged in azigzag fashion in the sub scanning direction of the image carrier.

FIG. 11 is an explanatory diagram showing an example of spot positions33Z formed on the image carrier by light emitting elements in case of anoptical head in which the light emitting elements are arranged in azigzag fashion. In FIG. 11, hatched portions U are pixels to be exposedto light from the light emitting elements. Portions V indicated by chaindouble-dashed lines are pixels not to be exposed to light from the lightemitting elements. “Pa” indicates a pixel pitch in the main scanningdirection and “Pb” indicates a pixel pitch in the sub scanningdirection. “Sa” indicates a spot position pitch in the main scanningdirection and “Sb” indicates a spot position pitch in the sub scanningdirection.

In the example of FIG. 11, either the pixel pitch Pa in the mainscanning direction and the pixel pitch Pb in the sub scanning directionis half of the spot position pitch. That is, the interval in the subscanning direction between spot positions to be formed on the imagecarrier by the light emitting elements is set to be twice of the pixelpitch in the sub scanning direction. The lines of spot positions aredivided into two groups: the first group 33 f, 33 h, 33 j, 33 l, 33 nand the second group 33 p, 33 r, 33 t, 33 v. In the first group and thesecond group, the positions of light emitting elements, i.e. pixels tobe exposed and pixels not to be exposed are alternatively arranged asseen in the main scanning direction.

FIG. 12 is a schematic block diagram of the driving of the lightemitting elements, corresponding to FIG. 11. Description will be made asregard to the light-emitting element line heads 28Y for yellow. Theshift resistors are divided into two groups: the first group 24 f-24 nand the second group 24 p-24 z. Among the shift resistors in the firstgroup, each of the shift resistors 24 f, 24 h, 24 j, and 24 l holds theimage data, outputs the image data to the light emitting elements, andtransmits the image data to the next shift resistor. The shift resistor24 n holds the image data and the outputs the image data to the lightemitting elements. Each of the shift resistors 24 g, 24 i, 24 k, and 24m of the first group holds the image data and transmits the image datato the next shift resistor without outputting the image data to thelight emitting elements.

Among the shift resistors of the second group, each of the shiftresistors 24 p, 24 r, 24 t, and 24 v holds the image data, outputs theimage data to the light emitting elements, and transmits the image datato the next shift resistor. The shift resistor 24 z holds the image dataand the outputs the image data to the light emitting elements. Each ofthe shift resistors 24 q, 24 s, 24 u, and 24 w hold the image data andtransmit the image data to the next shift resistor without outputtingthe image data to the light emitting elements.

In FIG. 12, the lines of the light emitting elements are divided intotwo groups: the first group 28 f, 28 h, 28 j, 28 l, 28 n and the secondgroup 28 p, 28 r, 28 t, 28 v, 28 z. The lines of the fist group arearranged to correspond to the spot positions of the first group shown inFIG. 11. In addition, the lines of the second group are arranged tocorrespond to the spot positions of the second group of FIG. 11.

Description will now be made as regard to the operation of the blockdiagram shown in FIG. 12. The data processing means 23 outputs imagedata to the shift resistors of the first group through a control line 35p, and outputs the image data to the shift resistors of the second groupthrough a control line 35 q. The image data is outputted from the shiftresistor 24 f to the light emitting elements of the first line 28 f ofthe first group so that the light emitting elements expose pixels at thespot positions 33 f on the image carrier. In addition, the image data isoutputted from the shift resistor 24 p to the light emitting elements ofthe first line 28 p of the second group so that the light emittingelements expose pixels at the spot positions 33 p on the image carrier.

As the image carrier moves for the pixel pitch pb in the sub scanningdirection, the image data is transmitted from the shift resistor 24 f tothe shift resistor 24 g of the first group. In addition, the image datais transmitted from the shift resistor 24 p to the shift resistor 24 qof the second group. At this time, the shift resistor 24 g and the shiftresistor 24 q do not output the image data to the light emittingelements so that no pixel is exposed. As the image carrier further movesfor the pixel pitch Pb in the sub scanning direction, the image data istransmitted from the shift resistor 24 g to the shift resistor 24 h. Inaddition, the image data is transmitted from the shift resistor 24 q tothe shift resistor 24 r. Then, the shift resistor 24 h and the shiftresistor 24 r output the image data to the lines 28 h and 28 r of thelight emitting elements, respectively. Accordingly, the same pixels inthe lines at the spot positions 33 f and 33 p are exposed to light.

After that, in the same manner, the movement of the image carrier, thetransmission of the image data to the respective shift resistors, andthe output of the image data to the light emitting elements aresequentially conducted, thereby conducting multiple exposure of eachsame pixel. In this case, the gradation control for neutral density canbe conducted on the basis of the data prepared by the data processingmeans 23.

Even when the light emitting elements are arranged in the zigzag fashionso that the interval in the sub scanning direction between the spotpositions where the light emitting elements emit light to the imagecarrier is an integral multiple of the pixel pitch in the sub scanningdirection, the multiple exposure of each pixel can be achieved byarranging the respective shift resistors which transmit the image datawithout outputting the image data as shown in FIG. 11 and FIG. 12.

As mentioned above, also in the optical head in which light emittingelements are arranged in a zigzag fashion, the storage means for pixellines and the lines light emitting elements can be arranged inone-on-one relationship. The timing for transmitting image data storedin a shift resistor to the next shift resistor and the timing for makinglight emitting elements in the line to emit light on the basis on theimage data stored in the shift resistor are synchronized, therebysimplifying the circuit structure and speeding up the operation.

The driving circuit of the active matrix method and the PWM control, andintensity modulation control as described with reference to FIG. 5-FIG.10 can be adopted to the case that the light emitting elements arearranged in the zigzag fashion as described above with reference to FIG.11 and FIG. 12. FIG. 14 is a block diagram showing an example in whichlight emitting elements are arranged in a zigzag fashion as shown inFIG. 11 and are controlled by the active matrix method as described withreference to FIG. 5. In this example, in addition to storage means(shift resistors 24 f-24 z) and lines of light emitting elements 28 f-28z, a line selector 34 is mounted on a line head 28A. “35 c” designates acontrol line for outputting image data from the data processing means 23to the shift resistors 24 p-24 z of the second group.

In FIG. 14, the line selector 34 and the shift resistors 24 f-24 ncontrol and drive light emitting parts arranged in the lines 28 h-28 naccording to the active matrix method as mentioned above. In addition,the line selector 34 and the shift resistors 24 p-24 z control and drivelight emitting parts arranged in the lines 28 p-28 z according to theactive matrix method. As shown in FIG. 14, since the driving controlaccording to the active matrix method can be applied even when the linesof light emitting elements are arranged in the zigzag fashion in the subscanning direction of the image carrier, the light emitting elements canbe maintained to emit light even during the image data is transmittedbetween the shift resistors, thereby exposing pixels with highluminance.

In the structure shown in FIG. 12, the shift resistors 24 f-24 z areformed separately from the lines of light emitting elements 28 f-28 z.However, in the present invention, the shift resistors 24 f-24 z may beformed on a same substrate as the lines of light emitting elements 28f-28 z as shown in the block diagram of FIG. 13. In this case, the lightemitting elements and the storage means can be manufactured together.The necessity of preparing the light emitting elements and the storagemeans on separate chips can be eliminated, thereby reducing themanufacturing cost.

Further, in the structure shown FIG. 12, a glass is used as theaforementioned substrate so that the storage means are composed of ThinFilm Transistors (TFTs). There are various methods of making the TFT.For example, silicon oxide is first deposited into a layer on a glasssubstrate and amorphous silicon is then deposited into a layer thereon.The amorphous silicon layer is exposed to excimer laser beam forcrystallization, thus forming a polysilicon layer as a channel. Afterpatterning the polysilicon layer, a gate insulating layer is formed andfurther a gate electrode is formed of tantalum nitride. Subsequently,source/drain regions for N-channel TFT are formed by ion implantation ofphosphorous and source/drain regions for P-channel TFT are formed by ionimplantation of boron.

After impurities of ion implantation is activated, a first layerinsulating film is formed, first contact holes are formed, source linesare formed, a second layer insulating film is formed, second contactholes are formed and metallic pixel electrodes are formed, therebycompleting the array of TFT (for example, see “Polymer Organic ELDisplay” presented at the 8^(th) Electronic Display Forum (Apr. 18,2001)). The metallic pixel electrode is made of metal such as Mg, Ag,Al, and Li and functions as a cathode for the organic EL emitter andalso as a reflection layer for the organic EL emitter.

Since the glass substrate having high dimensional stability isstructured and all of light emitting elements and storage elements areformed on the single substrate, improvement in accuracy of aiming lightfrom each light emitting element to each pixel on the image carrier isachieved.

In the present invention, organic EL (organic electroluminescence)arrays are employed in lines of light emitting elements for multipleexposure. FIG. 15 is a perspective view showing an example of theorganic EL array to be employed in the image forming apparatus of thepresent invention. In FIG. 15, an organic EL array 12 is mounted on arectangular substrate 1 made of glass or the like. The organic ELs areconnected to a driving circuit 11 for controlling the emission. Therectangular substrate 1 is provided with positioning pins 13 and screwholes 14 for installation formed on both sides thereof. “16” designatesa protective cover for covering the driving circuit 11 and the organicEL array 12. A condensing rod lens array 15 as magnifying optical systemis fixed on the side of the image carrier. Because of the condensingfunction of the condensing rod lens array 15, light-emitting parts ofthe organic EL array 12 are condensed to form an image on aphotosensitive surface of the image carrier.

FIG. 16 is a vertical sectional view showing an example of an organic ELarray head 10. In FIG. 16, a reflection layer 2 composed of dielectricmulti-layered film is formed on the substrate 1, made of glass or aresin film, by the spattering method. The reflection layer 2 composed ofa dielectric multi-layered film may be formed of, for example, a pair oflayers made of SiO₂ and TiO₂. The reflective layer 2 formed of such adielectric multi-layered film has reflectance of 0.99 or more. An anode3 is formed on the reflection layer 2 by the spattering method. Theanode 3 is made of a light-transmitting and conductive material. As anexample of the material having such characteristics, ITO (indium tinoxide) having large working function may be used.

Then, a hole transportation layer 4 is formed on the anode 3 by theinkjet method. After forming the hole transportation layer 4 within ahole 11, ink composition is discharged into the hole 8 from a head of aninkjet printing device, thereby achieving the patterning application onthe emitting layer of the pixel. After the application, the solvent isremoved and the applied ink composition is treated by heat, therebyforming a light-emitting layer 5.

The organic EL layer composed of the hole transportation layer 4 and theemitting layer 5 may be formed by other known method such as a spincoating method, a dipping method, and other liquid phase depositionmethod instead of applying ink compositions by inkjet method as theabove. The material of the hole transportation layer 4 and the emittinglayer 5 may be known EL materials listed in Japanese Patent UnexaminedPublication H10-12377 and Japanese Patent Unexamined Publication2000-323276, so description about details will be omitted. Then, acathode 6 is formed by vapor deposition method. As the material of thecathode 6, for example, Al may be employed.

The organic EL array head 10 has thin layer portions 6 a-6 c formed atthe cathodes 6 having a U-like section corresponding to light emittingparts 10 x-10 z. The thin layer portions 6 a-6 c are formed to have sucha thickness in holes of a wall 9 as to allow light transmission. At thelight emitting parts 10 x-10 z, semi-transparent reflection layers(dielectric mirrors) 7 composed of a plurality of dielectricmulti-layered films are formed on the bottoms of the cathodes 6 by thespattering method. The semi-transparent reflection layers 7 a-7 ccomposed of dielectric multi-layered films may be formed of, forexample, three pairs of layers made of SiO₂ and TiO₂. Thesemi-transparent reflection layers 7 formed of such dielectricmulti-layered films has reflectance of about 0.9.

In the embodiment of FIG. 16, as mentioned above, the thin layerportions 6 a-6 c are formed at the cathodes 6, thereby allowing lighttransmittance. Accordingly, even when the organic EL layer composed ofthe hole transportation layer 4 and the emitting layer 5 is formed by aliquid phase deposition method such as the inkjet method, it is freefrom the problem that the reflectance is reduced due to the smoothnessof contact portion between the EL layer and the cathode.

In the present invention, the organic EL array head having theaforementioned structure can be used as an exposure head of an imageforming apparatus, for example, capable of forming a color image byusing electrophotographic technique.

FIG. 17 is a front view schematically showing an example of an imageforming apparatus employing the organic EL array head described withreference to FIG. 15. The image forming apparatus is of a tandem type inwhich four similar organic EL array exposure heads 1K, 1C, 1M and 1Y aredisposed at the respective exposure positions of four similarphotosensitive drums (image carriers) 41K, 41C, 41M and 41Ycorresponding thereto. As shown in FIG. 17, the image forming apparatushas a driving roller 51, a driven roller 52, and a tension roller 53 andhas an intermediate transfer belt 50. The intermediate transfer belt 50is laid around the driving roller 51 and the driven roller 52 with acertain tension applied by the tension roller 53 and is driven tocirculate in the direction of the arrows shown in FIG. 17(counterclockwise direction) by the driving roller 51. Fourphotoreceptors 41K, 41C, 41M and 41Y are disposed at predetermineddistance along the intermediate transfer belt 50. Each photoreceptor hasa photosensitive layer on the outer peripheral surface thereof to serveas an image carrier.

Suffixes “K”, “C”, “I”, and “Y” added to reference numerals indicateblack, cyan, magenta, and yellow, respectively. That is, thephotoreceptors designated by reference numerals with such suffixes arephotoreceptors for black, cyan, magenta, and yellow, respectively. Thesame is true for other members. The photoreceptors 41K, 41C, 41M and 41Yare driven to rotate in the direction of arrows shown in FIG. 17(clockwise direction) synchronously with the driving of the intermediatetransfer belt 50. Arranged around each photoreceptor 41 (K, C, M, Y)area charging means (corona charger) 42 (K, C, M, Y) for uniformlycharging the outer peripheral surface of the photoreceptor 41 (K, C, M,Y), an organic EL array exposure head 1 (K, C, M, Y) having theaforementioned structure of the present invention for sequentiallyline-scanning the outer peripheral surface of the photoreceptor 41 (K,C, M, Y), which has been uniformly charged by the charging means 42 (K,C, M, Y), synchronously with the rotation of the photoreceptor 41 (K, C,M, Y).

Also arranged around each photoreceptor 41 (K, C, M, Y) are a developingdevice 44 (K, C, M, Y) for applying toner as a developer to anelectrostatic latent image formed by the organic EL array exposure head(K, C, M, Y) to form a visible image (toner image), a primary transferroller 45 (K, C, M, Y) serving as transfer means for sequentiallytransferring the toner image developed by the developing device 44 (K,C, M, Y) onto the intermediate transfer belt 50 as a primary transfertarget, and a cleaning device 46 (K, C, M, Y) as cleaning means forremoving the toner remaining on the surface of the photoreceptor 41 (K,C, M, Y) after the transfer of the toner image.

Each organic EL array exposure head 1 (K, C, M, Y) is installed in sucha manner that the array direction of the organic EL array exposure head1 (K, C, M, Y) is parallel to the bus-bar of the photoreceptor drum 41(K, C, M, Y). The emission energy peak wavelength of each organic ELarray exposure head 1 (K, C, M, Y) and the sensitivity peak wavelengthof the photo receptor 41 (K, C, M, Y) are set to be approximatelycoincident with each other. The developing device 44 (K, C, M, Y) usesan on-magnetic single-component toner as a developer, for example. Thesingle-component developer is conveyed to a development roller through asupply roller, for example, and the thickness of the developer layeradhering to the development roller surface is regulated with aregulating blade. The development roller is brought into contact with orpressed against the photoreceptor 41 (K, C, M, Y) to allow the developerto adhere to the surface of the photoreceptor 41 (K, C, M, Y) accordingto the electric potential level thereof, thereby developing theelectrostatic latent image into a toner image.

Toner images of black, cyan, magenta and yellow formed by unicolor tonerimage forming stations for the four colors are sequentially primarilytransferred on to the intermediate transfer belt 50 by a primarytransfer bias voltage applied to the respective primary transfer rollers45 (K, C, M, and Y), and sequentially superimposed on each other on theintermediate transfer belt 50 to form a full-color toner image, which isthen secondarily transferred onto a recording medium “P” such as a paperat a secondary transfer roller 66. The transferred full-color tonerimage is fixed on the recording medium “P” by passing between a pair offixing rollers 61 as a fixing device. Then, the recording medium “P” isdischarged through a pair of sheet delivery rollers 62 onto an outfeedtray 68 formed on the top of the apparatus body.

In FIG. 17, reference numeral 63 designates a sheet cassette in which astack of a large number of recording media “P” is held, 64 designates apickup roller for feeding the recording medium “P” from the sheetcassette 63 one by one, 65 designates a pair of gate rollers forregulating the timing at which each recording medium “P” is supplied tothe secondary transfer portion at a secondary transfer roller 66, 66designates the secondary transfer roller as a secondary transfer meansfor forming the secondary transfer portion together with theintermediate transfer belt 50, 67 designates a cleaning blade ascleaning means for removing the toner remaining on the surface of theintermediate transfer belt 50 after the secondary transfer. As mentionedabove, since the organic EL array is used as the writing means in theimage forming apparatus shown in FIG. 17, the apparatus can bemanufactured to have smaller size than a case using laser scanningoptical system.

Though the optical unit and the image forming apparatus of the presentinvention have been described with reference to the embodimentsdisclosed herein, the present invention is not limited thereto andvarious changes may be made therein.

1. An optical head comprising: light emitting elements which are alignedin a plurality of lines, extending in the main scanning direction of animage carrier and being arranged in parallel with each other in the subscanning direction of the image carrier, so that said light emittingelements are arranged in two-dimensional zigzag fashion; and storagemeans for storing image data and outputting the image data to said lightemitting elements, wherein: pixels on said image carrier are exposed bythe light emitting elements aligned in one line and exposed again by thelight emitting elements aligned in the next line after the movement ofsaid image carrier, and in the same manner, said pixels are sequentiallyexposed by the light emitting elements on another line after themovement of said image carrier so as to achieve multiple exposure of thepixels, pixels are capable of being exposed with gradational outputs,and pixels to be exposed by light emitting elements and pixels not to beexposed by light emitting elements are arranged in the zigzag fashion insaid sub scanning direction on said image carrier, and wherein saidstorage means comprise first storage means each of which outputs imagedata to light emitting elements in the corresponding line and secondstorage means each of which transfer the image data to the next firststorage means without outputting the image data to the light emittingelements.